At the Nintendo world event held in Tokyo, the company has revealed details about the battery life of their new handheld, the 3DS. They released a chart outlining the final hardware specs including the battery life of the system. It takes a 3DS three and a half hours to fully charge, which is nice. More importantly are the stats on how fast the battery drains. The following numbers are assuming you use the lower screen brightness level. On the lowest setting you'll get about 3-5 hours worth of time while playing a 3DS game. It goes up to 5-8 hours while playing a regular DS game. As the backlighting brightness levels go up, so the battery life goes down.
I've ever had my DS run down at an inopportune time while playing a game, but then I don't use it for long car/train/plane rides. The numbers do seem low, but then you have to consider how you will use it. This will be plenty on a normal daily commute and even on a cross country flight. In fact I'm really struggling to come up with another example other than a really long car ride where this battery wont last long enough without an outlet somewhere nearby.